This invention relates to the field of decomposition product purification, and in particular, to a method of purifying cumene hydroperoxide decomposition products from hydroxyacetone and from other carbonyl impurities.
It is well known that in a typical cumene-based process of acetone and phenol production, a number of impurities are formed along with the main product and the various byproducts. These impurities significantly complicate the stage of product distillation and furthermore have a substantial negative effect on the quality of desired final products. Hydroxyacetone (hereinafter "HA") and various carbonyl compounds such as aldehydes, are most prominent among such impurities. The presence of HA causes the formation of 2-methylbenzofurane (hereinafter "2MBF") and consequently results in worsening of the quality of the product phenol. Furthermore, aldehydes that are present in product acetone reduce acetone's stability to oxidation, further lowering the quality of the final product.
Despite the substantial difference in existing schemes of cleavage product distillation, most commonly some form of a technique that involves separate treatment of acetone and phenol streams with main and acidic reagents, is applied to obtain final acetone and phenol products of desired quality and purity.
Typically, treatment of crude acetone or acetone streams is implemented by adding an aqueous caustic solution which converts the aldehydes into deep condensation products having high boiling temperature. These high condensation products are not distilled along with product acetone, and thus it is possible to obtain product acetone with a desired stability to oxidation.
The main difficulty in converting aldehydes into condensation products via addition of a caustic solution to the acetone stream (or to the product acetone distillation columns), is the extremely low solubility of non-organic compounds (such as NaOH) that are catalysts of aldehyde conversion reactions to deep condensation products, in organic compounds such as acetone.
In order to achieve a high degree of acetone purity from aldehydes, a significant quantity of a caustic catalyst (such as NaOH) must be added during the process. This results in clogging and deterioration of distribution devices of columns and heat-exchanging equipment used in the process. An even more serious disadvantage of the existing method of purification, is that the desired product acetone undergoes a condensation reaction to form mesityl oxide (hereinafter "MO") and diacetone alcohol (hereinafter "DAA"). The condensation reaction is highly undesirable for two reasons. First, the reaction results in loss of desired product, and second, the reaction complicates the treatment procedure of acetone because the amount of MO and DM in product acetone is limited.
Thus, the must commonly applied method for acetone purification solves the problem of aldehydes removal but at the same time creates new problems--damage to process equipment caused by a caustic agent and required removal of undesired products formed as a result of treatment of acetone with the alkaline catalyst.
Purification of crude phenol or phenol stream is typically carried out by alkaline or acidic agents such as aqueous caustic solutions, amines or with an acidic catalytic treatment based on ion-exchange resins or zeolites. Aldehydes, hydroxyacetone and mesityl oxide are converted into deep condensation products under the effect of alkaline or acidic catalysts. The boiling temperature of these condensation products is higher than phenol. As a result, it is possible to separate the condensation products from phenol via distillation to obtain product phenol of required quality.
The most commonly used method of purifying crude phenol is via an acidic catalytic treatment which is usually accomplished with the use of various types of ion-exchange resins with high acidity. As a result, aldehydes, MO and other impurities present in phenol are converted into deep condensation products and then separated from phenol via distillation.
HA present in crude phenol reacts with phenol to form 2MBF which is nearly impossible to separate from phenol by distillation at a product phenol column. Furthermore, phenol customers typically demand that the amount of 2MBF in product phenol should not exceed 15 ppm. The amount of 2MBF formed with ion-exchange resins is determined first by the level of HA concentration in crude phenol delivered to ion-exchange resin treatment. At HA concentration &gt;30 ppm none of the existing ion-exchange resins are able to separate microimpurities from phenol in sufficient quantities. Thus phenol is typically purified from MO, alpha-methylstyrene and other impurities at the expense of undesirably forming 2MBF in amounts above the allowable limit.
A similar situation arises while using acidic zeolites for phenol treatment, as disclosed in commonly assigned U.S. Pat. No. 5,502,259 of Zakoshansky et al. Even though zeolites are superior to ion-exchange resins with respect to several parameters (i.e. a 4-8 times higher catalyst life, a degree of carbonyl purification that is at least twice better), they still do not completely solve the problem of a higher that desirable amount of 2MBF.
It should be noted that the amount of HA in a phenol stream arriving for purification is determined first by the level of overloading of cleavage products distillation columns, as well as by the amount of HA formed at the stage of cumene hydroperoxide cleavage.
Additionally, even a small increase in feed-rate (as low as 5% relative) causes the increase in the amount of HA delivered to the stage of acidic-catalytic purification several times (from 20-30% relative to 300-500% relative). Some variations of temperature profile or reflux flow to distillation column of phenol stream from acetone stream results in the same situation--increase in HA content in phenol stream and, hence, increase in 2MBF formed at the stage of acidic-catalytic purification.
Other attempts to solve the above-described problems have met with little success. For example, the technique of treating the phenol stream with a caustic solution as taught in U.S. Pat. No. 3,335,070 causes phenol to undergo a chemical reaction with the caustic solution to form a substantial amount of sodium phenate along with the formation of deep condensation products. This results in significant process overload at the dephenolation stage (i.e. conversion of sodium phenate to phenol) and also results in a great increase in waste water output. In another example, the technique of U.S. Pat. No. 3,692,845 proposes treating the phenol stream with amines for improved purification. However, during treatment of the phenol stream with amines, the products of amine reactions, along with aldehydes, hydroxyacetone and mesityl oxide, are removed from the process along with production waste phenol tar and then burnt. When the amine containing tar is burnt, nitrogen oxides are formed and released into the atmosphere creating undesirable levels of toxic pollution.
Thus, all the of the above-described currently implemented methods of phenol and acetone purification have a number of serious disadvantages resulting in difficulties in obtaining products of required quality.